1. FIELD OF THE INVENTION
The field of the invention is disc brake systems applicable mainly to automotive brake systems and more particularly is improvements in a motor disc brake system including a brake disc connected to a rotating member to be braked, a friction pad opposed to a side surface of the brake disc for advancing and retracting movements relative to the brake disc, a piston for pressing a back surface of the friction pad when the piston advances, a brake caliper mounted to a stationary bracket and carrying the piston such that the piston is capable of advancing and retracting, and a thrust generating device provided on the brake caliper and capable of generating a thrust to cause advancing and retracting movements of the piston, said thrust generating device comprising an electric motor, a drive shaft connected to an output shaft of the electric motor, a rotary shaft disposed coaxially with the piston and screw-fitted to the piston by a ball screw, and a reduction device disposed between the drive shaft and the rotary shaft to permit the drive shaft to drive the rotary shaft at a reduced speed.
2. DESCRIPTION OF THE PRIOR ART
In this type of disc brake device, it is conventionally arranged that the piston is directly carried on the inner peripheral surface of the cylinder bore of the brake caliper, as disclosed in Japanese patent publication Kokai No. 60-206766, for example. In this arrangement, when the electric motor is rotated in the normal direction and an advancing thrust is applied to the piston via the ball screw to bring the friction pad into pressure contact with the brake disc for braking action, a side thrust acts on the piston due to a brake reaction force coming from the brake disc and this increases the slide resistance of the piston rapidly, leading to an increase in the load of the electric motor. Consequently, it is required that an electric motor of a large capacity be employed in order to strengthen the braking force against such load increase.